This chapter deals with the peculiar safety issues linked to the tritium use in the tokamaks, in fact it is one of the most important safety concerns in this kind of devices. Being a gas with a low molecular mass has the ability to escape from confinement, to combine with other compounds and to tie with them depending on the affinity and/or the thermal dynamics conditions. Particular attention is posed to the accidents that could induce tritium release, as the ones affecting both the components of the fuel cycle in the tokamak plant and the vacuum vessel (VV) itself. The accidents in the VV are an issue because of the implantation of the tritium gas in the plasma facing components and structural materials occurring since the first plasmas with tritium as fuel. The safety criteria suggest the use at least of two strong confinement barriers around each tritium inventory. The process piping and equipments (e.g.: isotope separation system, molecular sieves, etc.) of the tritium facilities should be located inside a strong secondary containment (leak-tightened building served by dedicated ventilation and detritiation system). The tritium can affect significantly the occupational radiation exposure and the public safety as well. To avoid doses from the gas releases, its handling requests dedicated accuracy. In this chapter the main tritium source terms deriving from tokamaks and from the connected service systems, as fuel cycle, are discussed. The tritium contribution to the worker dose during normal plant operation and maintenance activities and the dose to the public in case of incidents or accidents are key issues in the plant performances and it is also discussed here. Biological hazards for workers and members of the public are linked to the radiological exposure to tritium which may occur by different pathways such as: inhalation, ingestion and absorption through the skin. External dose hazard is very unlikely, as the maximum energy of tritium beta particles is insufficient for skin penetration. A very important issue to determine the biological hazard from tritium exposure is its chemical and physical form. The relation between tritium exposure, radiological dose and health effects will be analysed in detail. Among the safety issues, the tritiated waste evaluation and treatment has a significant importance. In the fusion devices, the waste is mostly activated solid material due to neutron bombardment which affects mainly the plant components surrounding the plasma chamber where the n-flux is larger. Even though the products of the D-T fusion reaction (helium and neutrons) are not radioactive, high energy neutrons from fusion reactions modify some elements of structural materials producing radioactive isotopes. Furthermore, some tritium not reacting with deuterium, (only a small percentage of the D-T fuel injected in the plasma chamber is consumed during burn), and not extracted from the plasma chamber, could escape and contaminate the plasma facing components by various mechanisms (e.g.: diffusion, implantation and co-deposition). Hence, to summarize the main features of a large fraction of fusion waste it is possible to describe it as an activated solid which, in most cases, contains tritium that could introduce some complications to its management. A detritiation treatment prior to recycling or disposal is imperative for fusion components with high tritium content to fulfill regulatory requirements. The level of tritium contamination depends from the material physical and chemical properties, from its location with respect to the plasma chamber. © 2013 Nova Science Publishers, Inc. All rights reserved.
|Titolo:||Tritium in tokamak devices: Safety issues|
|Data di pubblicazione:||2013|
|Appare nelle tipologie:||2.1 Contributo in volume (Capitolo o Saggio)|